Q282 · Practice questionComplex familiar4 marks
QUESTION 282 (4 marks)
The partial particle-interaction diagram shows an incoming and outgoing electron on the left and an incoming and outgoing positron on the right. Time increases upward. This is the exchange/scattering channel, not annihilation.
a)[3 marks]
Complete the diagram with the exchanged boson. Add the conventional fermion arrows to all four external segments and label the boson.
b)[1 mark]
State the total lepton number before and after the scattering.
WORKED SOLUTION
4 marksPractice marking scheme
ANSWER
(a) A wavy exchanged photon connects the vertices; electron arrows follow increasing time and positron arrows oppose increasing time. (b) before and after.
Worked solution
(a) Use a photon for the electromagnetic exchange. Along the electron line, arrows go bottom-to-top. Along the positron line, the conventional fermion arrows run top-to-bottom, even though the incoming and outgoing event ordering is still set by the time axis.
(b) Each external state has with and with . The virtual photon has .
Equivalent physically justified methods, alternative valid sketches and consistent units accepted.
Accept sensible significant figures and consistent rounding from stated constants.
Part a: Draw a wavy photon line connecting the interaction vertices and label gamma.
Part a: Draw the electron arrows in the increasing-time direction.
Part a: Draw the positron fermion arrows against the increasing-time direction.
Part b: Give zero for both states.
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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